The mechanisms by which iron is stored, mobilized, and delivered in the human body are being examined. The structure of the metal binding sites of the iron transport protein, transferrin, is under active investigation. EPR and visible spectroscopy were used to establish that the anion necessary for transferrin to bind a metal ion is directly attached to the metal in the protein. The anion under physiologic conditions was shown to be bicarbonate and three tyrosines were shown to be involved in iron binding. A model for the three dimensional structure of the metal binding sites, based on the seven-coordinate structure of Fe(EDTA)H20- is proposed. Pulse labelling of rabbit reticulocytes with two radioisotopes of iron is being used to trace the path followed by iron between entering the red blood cell and being incorporated into hemoglobin.